IMAP: "BAD parse error: illegal character in quoted string"

Over the weekend, I upgraded my FOSS instance of Zimbra from 8.0.2 to 8.0.3. I've been using Zimbra for years on this one server, and it's worked great (sans a few mysterious issues, like the one I'm about to report).

Anyway, after the upgrade (which went fine), I noticed that IMAP now has a problem. I'm not sure if it's all users, some users, or just me, but my Apple Mail client suddenly began prompting me for the password over and over and saying it could not connect to the IMAP server. I have enabled logging with zmprov addAccountLogger myuser@mydomain.tld zimbra.imap debug, and began watching the mailbox.log file for ImapServer entries.

It seems that every time Apple Mail prompts me for my password and I enter it, I see this in the log immediately after:

During my several hours of troubleshooting, I decided to disable the Zimbra proxy and set the IMAP and POP3 services back to their normal ports. This caused the error to go away. Upon re-enabling the proxy, the problem is back. Does Nginx have its panties in a wad?

I also stumbled upon another thread here where setting zmlocalconfig -e nio_imap_enabled=false seemed to help a particular IMAP client, so I thought I'd try it. It worked, although Apple Mail continues to prompt me for my password at least mail was being read from the server. To verify, I set the property back to true, and the problem returned. So, I set the property back to false, but the problem remains, so apparently NIO doesn't have anything to do with it.

After two days of troubleshooting, I think I'm officially stumped. Ideas?

It seems the default setting for the Nginx proxy is to use literal strings during authentication instead of quoted string. But why do the log entries claim quoted strings? Is that Nginx complaining or is that the IMAP service itself?